The present invention pertains to a noise sequence generator and a CN controller using the generator. More particularly, the present invention relates to a noise sequence generator employing a technique for improving noise level and CN stability relative to time, and a CN controller using the generator.
In recent years, owing to the digitalization of the communications and broadcasting service industries, a variety of modulation systems are becoming available.
A method for evaluating the performance of equipment such as a digital receiver or a demodulator used in these digital systems includes investigating how a device bit error changes relative to changes in the CN ratio of an input signal.
In the case of performing such measurement, as its signal source, there is employed a reference signal generated by superposing a white Gaussian noise of an arbitrary level N as an interference wave on an ideal modulation signal of an arbitrary level C.
FIG. 8 shows a conventional reference signal generating system in accordance with an analog approach for generating this reference signal.
This reference signal generating system is composed of a digital modulation signal generator 1 and a CN controller 10.
That is, a digital modulation signal outputted from the digital modulation signal generator 1 in an RF bandwidth (a high frequency bandwidth) or an IF bandwidth (an intermediate frequency bandwidth) is adjusted at an arbitrary level C by means of a variable attenuator 11 of the CN controller 10, and then, the adjusted signal is inputted to one input terminal of a directional coupler 12.
In addition, a white noise signal outputted from a white noise generator 13 is bandwidth-restricted according to a frequency bandwidth of a digital modulation signal by means of a band pass filter 14. In addition, the bandwidth-restricted signal is adjusted to an arbitrary level N by means of a variable attenuator 15, and is then inputted to the other input terminal of the directional coupler 12.
Then, a reference signal R of an arbitrary CN ratio is outputted from this directional coupler 12.
In this reference signal generating system, the CN ratio of the reference signal R can be arbitrarily varied by adjusting the variable attenuators 11 and 15 of the CN controller 10.
The CN ratio (carrier to noise ratio) used here denotes a ratio of a carrier wave level to a noise level in a communication system for modulating and transmitting a carrier wave, as is well known.
In particular, in frequency modulation, it is known that, if the CN ratio is sufficiently large, a frequency deviation caused by noise is extremely small, and the SN ratio after demodulation is much better than a value of the CN ratio.
However, as in the aforementioned conventional CN controller 10, in a system for attenuating a digital modulation signal of an analog type in an RF bandwidth or IF bandwidth and a noise signal of an analog type respectively by an attenuator to be superposed, there has been a problem that the reliability of setting the CN ratio of the reference signal R is low.
That is, in the digital modulation signal generator 1, the digital modulation signal and carrier signal in a base bandwidth are inputted to an quadrature modulator, whereby the digital modulation signal in the RF bandwidth or IF bandwidth is generated.
In this case, under the influence of a level change of a carrier signal or frequency characteristics of the quadrature modulator, the reliability of the level of the digital modulation signal generated by the digital modulation signal generator 1 is lowered, thereby lowering the reliability of the CN ratio of the reference signal set at the CN controller 10.
In addition, the level of a white noise signal outputted from an analog white noise generator 13 is also likely to change with an elapse of time, and the reliability of the CN ratio of the reference signal R is lowered by this change.
In order to solve this problem, it has been considered to superpose a digital white noise sequence onto a baseband digital modulation signal, and then, convert it in the RF bandwidth or IF bandwidth.
However, as described previously, in the case of performing error measurement for evaluation of equipment performance, much time is required (about 10 hours to several days).
In a conventional technique, it has been extremely difficult to digitally generate a noise sequence free of correlation over such a long time.
For example, in the case of reading out a pre-stored noise sequence from a storage medium, in order for a noise sequence to be read out by one cycle within about 12 days when its reading clock is 32 MHz, an address region of about 35xc3x971012 is required.
In addition, in the case of performing readout in units of 2 bytes (16 bits), a storage medium with a storage capacity of about 70xc3x97103 Gigabytes is required.
In the case where an attempt is made to configure such a large scale system with hard disks, 10000 hard disks, each of 7 Gigabytes would be required, and thus, it is extremely difficult to achieve such a configuration.
The present invention has been made in order to solve the foregoing problem. It is an object of the present invention to provide a noise sequence generator of a simple configuration, for generating a noise sequence whose code period is long, whose level stability is high, and which is close to a Gaussian normal distribution; and a CN controller capable of increasing the reliability of the CN ratio of a reference signal relevant to the CN ratio set by using this noise sequence generator.
In order to achieve the foregoing object, according to a first aspect of the present invention, there is provided a noise sequence generator comprising:
a plurality of memories (31a, 31b, 31c, 32a, 32b, and 32c) storing plural types of noise sequences that are non-correlated with each other, respectively;
noise sequence readout means for reading out the noise sequences stored in the plurality of memories so that their code periods are prime numbers each other; and
sequence adding means (38, 39) for adding noise sequences read out from the plurality of memories in parallel by the noise sequence readout means, thereby outputting the addition result as a series of noise sequences.
In addition, in order to achieve the foregoing object, according to a second aspect of the present invention, there is provided a CN controller comprising:
a register (23);
setting means (24) for setting a value to determine an amplitude of a digital modulation signal row to the register;
multiplying means (21, 22) for multiplying the setting value of the register by a to-be-inputted digital modulation signal row;
a plurality of memories (31a, 31b, 31c, 32a, 32b, and 32c) storing white Gaussian noise sequences that are non-correlated with each other, respectively;
noise sequence readout means for reading out the noise sequences stored in the plurality of memories so that their code periods are prime numbers of each other (37);
sequence adding means (38, 39) for adding noise sequences read out in parallel from the plurality of memories by the noise sequence readout means, thereby outputting the addition result as a series of noise sequences; and
CN adding means (25, 26) for adding and outputting a series of noise sequences outputted from the sequence adding means to an output from the multiplying means.
In addition, in order to achieve the foregoing object, according to a third aspect of the present invention, there is provided a CN controller comprising:
a plurality of memories (31a, 31b, 31c, 32a, 32b, and 32c) storing white Gaussian noise sequences that are non-correlated with each other, respectively;
noise sequence readout means for reading out the noise sequences stored in the plurality of memories so that their code periods are prime numbers of each other (37);
sequence adding means (38, 39) for adding noise sequences read out in parallel from the plurality of memories by the noise sequence readout means, thereby outputting the addition result as a series of noise sequences;
a register (23);
setting means (24) for setting a value to determine an amplitude of the series of noise sequences to the register;
multiplying means (21, 22) for multiplying the setting value of the register to the series of noise sequences outputted from the sequence adding means; and
CN adding means (25, 26) for adding a to-be-inputted digital modulation signal row to an output from the multiplying means, and outputting the addition result.
In addition, in order to achieve the foregoing object, according to a fourth aspect of the present invention, there is provided a CN controller comprising:
a noise sequence generator (30) comprising:
a plurality of memories (31a, 31b, 31c, 32a, 32b, and 32c) storing a plurality of sequences that are non-correlated with each other, respectively;
noise sequence readout means for reading out a plurality of noise sequences stored in the plurality of memories so that their code periods are prime numbers of each other (37); and
sequence adding means (38, 39) for adding a plurality of noise sequences read out in parallel from the plurality of memories by the noise sequence readout means, thereby outputting the addition result as a series of noise sequences; and
CN control means (21, 22, 23, 24, 25, and 26) for receiving the series of noise sequences outputted from the sequence adding means of the noise sequence generator at one end, and receiving a digital modulation signal row to be CN controlled at the other end, thereby applying a predetermined CN control to the to-be-CN controlled digital modulation signal row based on the series of noise sequences.